1. Field of the Invention
This invention relates to a method of producing a hollow fiber membrane of ethylene-vinyl alcohol (EVA) copolymer. More particularly, this invention relates to a method of producing an EVA hollow fiber membrane having a uniform homogeneous porous structure by dry-wet spinning techniques.
2. Description of the Prior Art
A variety of hollow fiber membranes have been proposed as medical and industrial dialysis membranes and as ultrafiltration membranes. The present inventors have heretofore developed EVA copolymer hollow fiber membranes having satisfactory biocompatibility, improved durability and superior chemical stability, and have also developed an EVA hollow fiber membrane with a homogeneous structure.
The above EVA hollow fiber membrane, as disclosed in U.S. Pat. No. 4,134,837, displays excellent performance characteristics as a dialysis membrane and is being employed in artificial kidneys. The homogeneous EVA hollow fiber membrane is characterized by an inter-bonding of a plurality of particles having an average diameter of 100 to 10,000 Angstrom units and a substantial absence of voids sized in excess of 2.mu..
Generally, in addition to the homogeneous membrane such as described above, there is also known an anisotropic membrane which has a skin or active layer. Most of the previously known anisotropic hollow fiber membranes have been manufactured from cellulose acetate and polyacrylonitrile; however, a few of the prior anisotropic membranes have been made from EVA polymer. Japanese Patent Application Laid Open No. 77883/53 (1978) discloses an EVA anisotropic membrane for use as a separator for storage batteries. This membrane comprises a dense layer having an average pore diameter no larger than 300 Angstrom units and a porous layer having an average pore diameter of 0.05 to 10.mu.. The membrane is manufactured by casting a polymer solution onto a glass plate, allowing it to stand in the air and, then coagulating it in a coagulation bath. This means that the above method provides only a flat sheet product and does not provide a hollow fiber membrane such as contemplated by this invention.
The present inventors conducted studies on the technology for producing various selectively permeable membranes, particularly hollow fibers, and have recognized that the production technology for a flat film and that for a hollow fiber involve quite different problems. By way of illustration, in the case of a hollow fiber membrane, the polymer solution is continuously extruded from a spinning nozzle and the fiber so formed must be coagulated and shaped into a membrane in a continuous sequence and in a limited period of time. This means that consideration must be paid not only to the membrane-forming ability of the polymer solution itself but also to a number of coagulation parameters and conditions. Furthermore, our research has shown that EVA copolymers are characterized by a large variation in membrane porosity according to coagulating conditions so that on EVA membranes, ranging in structure from those having a uniform micropore structure, which is the object of the present invention, to those having a porous supporting layer with a plurality of large vacuoles therein can be obtained.
Our studies on the above characteristics of EVA copolymers led to the instant invention which provides a new method for manufacturing a hollow fiber membrane having a uniform porous structure with good reproducibility.